In recent years, membrane technology has been extensively investigated for water treatment. In addition to removing particulates, some membrane processes that use membranes with very small pores are able to remove a significant amount of natural organic matter (NOM); however, the NOM that is removed often contributes to fouling of the membranes, even when only a small fraction of the NOM in the feed solution is removed.
In conventional membrane systems, molecules that are larger than the pore size of the membrane and those that adsorb to the membrane surface are retained, while other molecules pass through the membrane along with the permeate flow. It has been proposed that powdered activated carbon (PAC) be used in conjunction with membranes to adsorb a portion of the NOM. Then, the membrane rejects the PAC particles and the adsorbed NOM. In this way, fewer NOM molecules remain in solution and contact the membrane directly. While the NOM removal efficiency of such a combined process can be quite high, membrane fouling is often not reduced proportionately, and in some cases it is even exacerbated.
In addition to powdered activated carbon, it has been proposed that freshly precipitated iron oxides (FeOOH or Fe2O3.nH2O) or aluminum oxides (Al2O3) be used in conjunction with membranes to reduce fouling of the membrane. Like PAC, these freshly precipitated oxides sometimes reduce fouling and sometimes exacerbate it. Also, if the oxides accumulate on the membrane until they form a continuous layer, that layer can impart a very large hydraulic resistance, making it almost impossible for water to pass through the layer and reach the membrane.
U.S. Pat. No. 6,113,792 describes heating precipitated iron oxide particles in an oven to produce heated iron oxide particles useful with membrane filtration to remove contaminants from water.
With the ever increasing concern about the quality of drinking water, there continues to be a need for improved systems for effectively and economically removing contaminants such as natural organic matter from liquids. Such systems should provide effective removal of contaminants from the liquids and reliably reduce membrane fouling.